1. Field of the Invention
The present invention relates to audio apparatuses. In particular, the present invention relates to an audio apparatus for reproducing stereo signals down-mixed in two channels containing surround-sound components.
2. Description of the Related Art
With the widespread proliferation of terrestrial digital broadcast, 5.1-channel surround-sound broadcast is expected to increase in the future. 5.1-channel surround-sound broadcast can be fully available in only areas where full segments (12 or 13 segments) can be received. Vehicle-mounted apparatuses, which are mounted on moving objects, change in their reception environments as a result of movement of the moving objects, and thus have difficulty in maintaining 5.1-channel full-segment reception. When the moving object leaves a reception area that employs the full segment scheme, the reception is automatically switched to one-segment broadcast reception. As a result, the vehicle-mounted apparatus changes its reproduction operation from 5.1-channel surround-sound reproduction to 2.1-channel stereo-sound reproduction. Thus, the reproduced sound quality changes significantly. For the vehicle-mounted apparatus, therefore, down-mixed 2-channel stereo sound output is preferable for the tuner thereof, even during reception of 5.1-channel surround-sound broadcast. With this arrangement, even when the reception of the moving object is switched to one-segment, the stereo sound output is maintained, and thus, a large-scale change in the audio system can be restricted.
Currently, moving objects that have 5.1-channel surround speakers for entertainment enhancement are not uncommon. Thus, it is also desired that surround-sound signals be generated from down-mixed stereo signals and be reproduced as 5.1 channel-surround sound, even when stereo signals in which the down-mixed 5.1-channel surround-sound is mixed are reproduced.
For example, Japanese Patent No. 3682032 discloses a technology for generating surround-sound signals from 2-channel stereo signals. In this technology, an adaptive filter is used to extract components that are highly correlated with R signals in L signals of input stereo signals, and the extracted components are subtracted from the L signals to generate surround-sound signals SL. Similarly, components that are highly correlated with the L signals in the R signals of the input stereo signals are extracted, and the extracted components are subtracted from the R signals to generate surround-sound signals SR. This provides decorrelated surround-sound signals SL and SR.
As described above, the vehicle-mounted apparatus can also down-mix 1-channel surround-sound signals into 2-channel stereo signals and reproduce the resulting signals. In addition, a scheme in which down-mixed stereo signals are transmitted by a broadcast station is also available. For example, the ARIB Standard (described in ARIB STD-B21 6.2) defines a case of down-mixing 5.1-channel surround sound into 2-channel sound, as shown in FIGS. 1A and 1B. FIG. 1A illustrates stereo signals Lt and Rt in the absence of a pseudo surround flag, and FIG. 1B illustrates stereo signals Lt and Rt in the presence of the pseudo surround flag. Lt and Rt indicate stereo signals, Sl and Sr indicate surround-sound signals, and C indicates signals for a center speaker.
A broadcast station or a creator that creates audio data encodes down-mixed stereo signals Lt and Rt containing surround-sound signals Sl and Sr in accordance with a predetermined algorithm and transmits the encoded signals. A receiver decodes the encoded data stream to reproduce the down-mixed stereo signals Lt and Rt. The encoded data stream contains a pseudo surround enable signal, and the presence/absence of pseudo surround sound is identified based on the logic high or logic low of the enable signal. The data stream further contains a flag (a parameter k) for identifying a ratio of contained surround-sound signals Sl and Sr. For example, in the absence of a pseudo surround flag, as shown in FIG. 1A, the parameter k is 1/√2 for a flag “0” and the parameter k is ½ for a flag “1”.
When the parameter k is 0 in the equations 2.2.1 and 2.1.2 shown in FIGS. 1A and 1B, the stereo signals Lt and Rt are given as equations 2.3.1 and 2.3.2 below:
                    Lt        =                  L          +                                    1                              2                                      ×            C                                              (                  2.3          ⁢          .1                )                                Rt        =                  R          +                                    1                              2                                      ×            C                                              (                  2.3          ⁢          .2                )            
When the signals Lt and Rt are assumed to be typical stereo signals, a cross-correlation coefficient between the two signals is statistically given as an average of about 0.7. FIG. 2 is a graph showing cross-correlation coefficients of stereo signals. The typical stereo signals described above exhibit a line L1. Since the C (center) signals are added to the L signals and R signals in equations 2.3.1 and 2.3.2 noted above, the cross-correlation coefficient increases relatively and exhibits a line L2, which has a higher cross-correlation coefficient than the line L1. For comparison, when the signals Lt and Rt are the same (i.e., mono), the cross-correlation coefficient is 1.0. In this state, how the surround-sound signals Sl and Sr are mixed is expressed by equations 2.1.1 and 2.1.2. In surround-sound creation, the correlation between signals L and signals Sl is low and the correlation between signals R and signals Sr is also low, That is, when Sl and Sr are added to equations 2.3.1 and 2.3.2, respectively, the cross-correlation coefficient between Lt and Rt decreases. The ratio of the addition is further changed by the value of a parameter k (the flag value: matrix_mixdown_idx), and the cross-correlation coefficient between Lt and Rt changes. In the graph shown in FIG. 2, a line L3 represents a cross-correlation coefficient for the parameter k=½, and it is shown that the cross-correlation coefficient is smaller than that of the line L1 for typical stereo signals.
When a vehicle-mounted apparatus performs decorrelation processing by using stereo signals Lt and Rt generated by the down-mix scheme, a change of the cross-correlation coefficient (i.e., a change of the parameter k) also causes a change in the outputs (i.e., low-correlation components) of decorrelated surround-sound signals. A change in the output level occurs depending on whether the cross-correlation coefficient is large or small. That is, when the cross-correlation coefficient is large, the output level of the decorrelated surround-sound signals decreases, and when the cross-correlation coefficient is small, the output level of the decorrelated surround-sound signals increases. In order for a listener to maintain a homogeneous output level, it is necessary to control the decorrelation processing by using the parameter k of the terrestrial digital receiver.